Fix wait schema agreement

[dkropachev]

Pre-review checklist

Make schema agreement waiting code renew connection on each iteration

When schema agreement is started it could happen that control connection is getting disconnected/reconnected, when it happens schema agreement code used to use disconnected connection to run all the queries. As result, it could lead to schema agreement timeout, even if all nodes got schema updated long time ago.

This commit updates connection on every iteration and makes it iterate when underlying connection is closed

Fixes: https://github.com/scylladb/python-driver/issues/458

• [x] I have split my patch into logically separate commits.
• [x] All commit messages clearly explain what they change and why.
• [ ] ~~I added relevant tests for new features and bug fixes.~~
• [x] All commits compile, pass static checks and pass test.
• [x] PR description sums up the changes and reasons why they should be introduced.
• [ ] ~~I have provided docstrings for the public items that I want to introduce.~~
• [ ] ~~I have adjusted the documentation in ./docs/source/.~~
• [x] I added appropriate Fixes: annotations to PR description.